The present invention relates generally to a gas distributor useful for a vapor coating method and container. The present invention particularly relates to a gas distributor for introducing nonoxidizing or inert carrier gases for vapor coating of articles such as gas turbine engine blades with a metallic coating, especially an aluminide coating.
Certain articles operating at elevated temperatures in an oxidizing atmosphere have been provided with environmental protection in the form of coatings of various types. For example, components such as gas turbine engine turbine blades, vanes and other airfoils operating at high temperatures typically experienced in the turbine section of the engine frequently include metallic surface coatings alone or in various combinations with other materials. Such coatings are capable of resisting the oxidation, corrosion and sulfidation conditions generated during high temperature operation.
Application methods for such metallic coatings include depositing a vapor of one or more protective metals, for example aluminum or alloys of aluminum, to provide a form of aluminide coating, on an article surface at high temperatures. Such vapor coating methods are typically conducted in a nonoxidizing or inert atmosphere (e.g. hydrogen, nitrogen, helium or argon) within a coating container or chamber commonly referred to as a “retort”. Generally, the article or more typically articles (e.g., airfoils such as turbine blades) to be coated are placed within the container, along with a source of the aluminide coating, typically in the form of metallic pellets or powder, and is often retained in perforated baskets that can be arranged in rows to surround the articles. The container is then placed within a heater such as a furnace to generate a coating vapor. Generation of the coating vapor typically includes the use of halide “activators” such as fluorides, chlorides or bromides. This halide activator can be in the form of a gas that is introduced into the container to react with the source of the aluminide coating to form the aluminide-bearing gas or can be generated from a halide activator source within the container that forms the reactive halide gas upon heating.
The aluminide-bearing gas is typically transported or moved within the coating container by a nonoxidizing or inert carrier gas (e.g., hydrogen, nitrogen, helium or argon). In some vapor coating systems, this carrier gas is introduced through the bottom of the container and carries the aluminide-bearing gas upwardly to coat the articles. See, for example, U.S. Pat. No. 4,148,275 (Benden et al), issued Apr. 10, 1979; U.S. Pat. No. 5,928,725 (Howard et al), issued Jul. 27, 1999. In other vapor coating systems, the carrier gas is introduced through the top of the coating container and then diffuses throughout the container to carry the aluminide-bearing gas and coat the articles. See U.S. Pat. No. 6,039,810 (Mantkowski et al), issued Mar. 21, 2000. The advantage in introducing a carrier gas, such as argon, at the top (versus the bottom) of the container is that argon, being denser and heavier than air, will naturally flow downwardly through the container to commingle with the metallic (aluminide) coating vapor and will also act as a “plunger” to aid in the internal coating of the articles.
In one such system where the carrier gas is introduced through the top of the container, a gas distributor is used to disperse the carrier gas. One such gas distributor has a configuration similar to that of a “shower head” in that it is provided with a plurality of gas outlet holes spaced along the periphery of the cylindrical or disk-shaped head through which the carrier gas exits. This “shower head” distributor is typically positioned at the top of the container and above the aluminide generating pellets and articles to be coated.
It has been found that when a carrier gas such as argon is introduced through such a “shower head” distributor at the top of the container, the aluminide-bearing gas is not consistently moved or mixed within the coating container. This is particularly true as the argon gas moves and diffuses through the rows of aluminide generating pellets and through the rows of articles (e.g., airfoils) to be coated. Because the rows of pellets and articles impede or resist the gas flow, regions having varying densities of aluminide-bearing gas can be formed, thus creating a nonhomogeneous environment of the aluminide-bearing gas surrounding the articles to be coated. This nonhomogeneous environment of the aluminide-bearing gas usually results in an inconsistent distribution of the aluminide coating on the exterior of the article, as well as inconsistent internal gas flow and coating of the interior surface of the article (e.g. hollow airfoils such as hollow gas turbine blades).
Accordingly, it would be desirable to be able to provide a gas distributor that can introduce the carrier gas in a manner such that the aluminide-bearing gas is consistently moved and mixed within the coating container such that a more uniform and consistent aluminide coating is provided on the exterior of the articles, as well as on the interior of hollow articles.